J. Wüstehube “Schaltnetzteile: Grundlagen, Entwurf, Schaltungsbeispiele” [Switched mode power supplies: fundamentals, design, circuit examples], Grafenau/Württemberg, Expert-Verlag, 1982, cf. FIG. 13.3 therein, has disclosed a 12 W flyback converter switched mode power supply having a DC voltage connection, at which a DC voltage can be made available, a switching unit which has a supply voltage connection, the DC voltage connection being coupled to the supply voltage connection of the switching unit, and a first capacitor being coupled between the supply voltage connection and a reference potential, an AC voltage connection, at which a voltage can be made available which has at least one AC component, the series circuit comprising a first nonreactive resistor and a first diode as well as a second nonreactive resistor being coupled between the AC voltage connection and the supply voltage connection, a connection point being defined between the first diode and the second nonreactive resistor, and a second capacitor being coupled between said connection point and the reference potential. However, since this switched mode power supply has a different purpose than an electronic ballast according to the invention, in particular the relevant part of the circuit therein envisages another function to that of the electronic ballast according to the invention, cf. the embodiments relating to this further below, reference is initially made to the circuit arrangement illustrated in FIG. 1, which is known from the prior art and which is implemented in an electronic ballast, in order to illustrate the problem on which the invention is based:
Modern electronic ballasts for lamps are equipped with open-loop and/or closed-loop control electronics which comprise an ASIC or a microcontroller. Such components generally require a voltage supply which is stable in all operating modes and even at a low system voltage and should be designed so as to be as low-loss as possible in the electronic ballast. In FIG. 1, the switching unit of the open-loop and/or closed-loop control electronics is denoted by 10. At its supply voltage connection 12, into which a current IS flows during operation of the switching unit 10, two supply paths are coupled, namely from a DC voltage source at the connection 14 and from an AC voltage source at the connection 16. In this case, a capacitor C1 is slowly charged via a highly resistive resistor R3 from the DC voltage source 14, which in particular makes available the rectified system voltage. R3 is to be designed to have a high resistance value since a low-resistance design would lead to high permanent losses and thus to high temperatures at the resistor R3 during operation. The capacitor C1 is moreover charged from an internal AC voltage source of the electronic ballast, for example the PFC (power factor correction) control winding or the trapezoidal capacitor, via the series circuit comprising a nonreactive resistor R1 and a diode D1. Owing to the diode D1, the AC voltage signal at the connection 16 is rectified. The zener diode D3 at the supply connection 12 of the switching unit 10 limits the voltage across the capacitor C1 to the maximum permissible value for the switching unit 10. At times when there is an excessive supply of current, i.e. in particular in the case of an AC voltage having a high amplitude at the connection 16, this energy is not stored in the capacitor C1 but is diverted, via the zener diode D3, to the reference potential and converted into heat. At times when there is a low supply of energy via the AC voltage connection 16, there is the risk of the energy which can be made available via the DC voltage connection 14 being insufficient for supplying the switching unit 10, i.e. the voltage potential at the input 12 of the switching unit 10 is reduced to below the permissible minimum value. In this context, reference is made to FIG. 3, which shows the temporal voltage profile at the connection 12 of the switching unit 10 for the circuit arrangement illustrated in FIG. 1. A minimum voltage of 15 V should generally not be undershot. As shown in FIG. 3, the voltage UC1 is reduced temporarily down to 13.08 V.